Unbreak the powerpc/powerpc64 tinderbox

PR: 198805
Submitted by: sbruno

MFC r280430:

r280430 (by bz):

Make ix_crcstrip a public symbol for the moment; it probably is not
the right solution but I will leave it to experts to untangle this
problem to properly stop the build failures.

At the moment only if_ix.c includes dev/netmap/ixgbe_netmap.h which is
good as ixgbe_netmap.h defines a couple of (file) static variables--thus
local to if_ix.c.
static int ix_crcstrip however now also got checked from ix_txrx.c
(as an extern) and should not be visible there. In fact we do see
powerpc and powerpc64 build failures because of this. It is unclear
to me why on other (clang built?) architectures this does not lead
to a reference of an undefined symbol and similar build breakage.